1. Field of Invention
The invention relates to a moisturizer for the bipolar plate of a fuel cell and more particularly to a proton exchange membrane fuel cell.
2. Related Art
Energy is the most basic thing in human life. With the advance in technology, human beings make use of all kinds of energy to improve their life and change the history.
However, the utilization of various kinds of energy causes damages to the environment, such as the green house effect and air, water, and soil pollution. Therefore, it is imperative to develop a highly efficient but clean energy sources. In 1839, the English judge William Graff discovered the fuel cell in a private amateur research project. A hundred years later, it is selected as the important power supply in the U.S. space project. In 1965, it is used in Gemini V in its voyage.
The fuel cell has several features that are suitable for modern world. The first is its efficiency. It has a very high energy conversion efficiency, above 40%. If one can recycle the exhaust heat released during its electrochemical reaction, the efficiency can be over 80%. The second feature is its cleanness. It almost produces no pollution at all during the power supply process. Take the largest fuel cell plant with the capacity of 11 megawatts (MW) as an example. It also uses the natural gas as its fuel. The nitrogen-oxygen concentration produced by the plant during its early run is 1 ppm. No sulfur-oxygen compounds and particle pollutants are detected. It is cleaner than the cleanest power plant of other kinds in the world. The third feature is its quietness. The noise in the neighborhood of the 11 MW is below 55 decibel (dB). The fuel cell has wide applications, ranging from power, industrial, transportation, space, and military fields. There are many kinds of products made based upon the idea of fuel cells. Examples are power plants, spare batteries, electric cars, small submarines, and even the power supply for space ships and space shuttles.
The most common fuel cell is the proton exchange membrane fuel cell (PEMFC), also called the polymer membrane fuel cell. The cross section of its single cell is shown in FIG. 1. It is made of a membrane electrode assembly (MEA) 10 sandwiched between two bipolar plates 20.
The MEA 10 is its central part. It has five layers: the anode gas diffusion layer, the anode catalyst layer, the electrolyte layer (proton exchange membrane), the cathode catalyst layer, and the cathode gas diffusion layer. The proton exchange membrane can be a polymer membrane, such as Nafion produced by DuPont. The surface of the proton exchange membrane is further coated with a layer of catalyst and attached with a carbon cloth or paper as the catalyst layer and the gas diffusion layer, respectively.
The bipolar plate 20 is made of an electrically conductive material, such as graphite. Both of its surfaces are engraved with gas channels as the fuel gas channel 21 and oxidant channel 22 of the anode and cathode.
To generate energy, the fuel gas and the oxidant are guided into the fuel channel 21 of the anode and the oxidant channel 22 of the cathode. The fuel gas undergoes an oxidation process with the catalyst on the anode and releases electrons, producing hydrogen ions. The released electrons go out through a circuit to provide the desired current and finally enter the cathode to recombine with the hydrogen ions that pass through the electrolyte. The hydrogen then undergoes a reduction process with the oxygen atoms in the oxidant to form water.
In the use of fuel cells, the fuel gas is normally the hydrogen molecule or a reformate rich in hydrogen. The oxidant is usually oxygen molecules, but air is also used in practice.
To increase or adjust the current and power output from the fuel cell, several single cells can be connected in series to form a cell stack, as shown in FIG. 2. Surrounding the cell stack are collectors 30 and end boards 40. The collector 30 collects the current produced by the complete cell stack. The end boards 40 on both sides have a fuel inlet 41, an oxidant inlet 42, an oxidant outlet 43, and a fuel outlet 44.
In the PEMFC, the current and power produced in the electrochemical reaction determine the efficiency of the fuel cell. Factors that determine the produced current and power include: (1) the design of the fuel channel 21 and the oxidant channel 22 on the bipolar plates 20; (2) the effective area of the catalyst on the MEA 10 surfaces; (3) the material of the proton exchange membrane; and (4) the thickness and gas of the electrode layers.
The main function of the proton exchange membrane in the MEA 10 is to prevent reaction gases, hydrogen and oxygen, from crossing over. It simultaneously blocks electrons but allows hydrogen ions (protons) to enter the cathode side from the anode side in the fuel cell. Therefore, it achieves the same effect as a bridge. However, the transportation of protons in the proton exchange membrane requires water molecules as the medium. Under the electro-osmotic drag, the protons move from the anode to the cathode, forcing water molecules to move in the same direction too. This will result in the proton exchange membrane's drying. The internal resistance of the fuel cell therefore goes up and the efficiency thus decreases.
According to experiments and analyses, when each proton is accompanied by more water molecules and they pass through the proton exchange membrane together, the voltage generated by the fuel cell will be larger as the resistance of the proton gets lower. Therefore, adding appropriate amount of water can effectively increase the efficiency of the fuel cell.
Since each hydrogen molecule can be ionized into two protons, the transportation rate at the proton exchange membrane is limited by its saturation density. Thus, even if one makes the hydrogen gas reach its saturation humidity, the number of water molecules that can travel with each proton is still restricted.
Consequently, it is not sufficient to simply moisturize the fuel at the fuel inlet. This can only make the fuel gas moisturized in the beginning. After passing through the whole fuel channel 21, the water molecules become so few that the protons still have difficulty passing through the proton exchange membrane.
Thus, the fuel cell usually has a lower internal resistance in the wetter region of the fuel channel 21 but a higher on in the dryer region. The net result is that the current density in the early section of the hydrogen incoming channel is higher than that in the later section. How to moisturize the fuel gas in the middle and later sections of the fuel channel 21 in order to lower the internal resistance of the dryer region is a crucial condition to maintain the PEMFC.
Some propose to directly add water supply at the inlet of the fuel channel 21 to moisturize the fuel gas. Although this method can indeed increase the humidity of the fuel gas at the inlet, it inevitably causes many other problems.
First, adding water at the inlet of the fuel channel 21 is likely to flood the gas diffusion layer on the anode if no proper solution is provided. When the gas diffusion layer is flooded with water, the path for the hydrogen to diffuse to the catalyst layer is clogged, resulting in a bad efficiency of the fuel cell.
Secondly, adding water at the inlet of the fuel channel 21 cannot effectively improve the dry situation in the middle and later sections of the fuel channel 21. Therefore, it is not a perfect method.